Almost a quarter of the GOP's nominating delegates are at stake in the March 1 contests.

Speaking Sunday to CNN, Cruz cast Trump as a carbon copy of Democratic presidential front-runner Hillary Clinton and suggested that not even Trump "knows what he would do" as president.

Trump, meanwhile, is mocking Republican leaders who say his nomination would be catastrophic for the party.

Also on CNN, the real estate mogul rejected calls to release his tax returns, saying he's already disclosed his personal financial records. Trump also declined to denounce a former KKK leader, David Duke, who publicly backs his bid.